Ubuntu and Debian use the alternatives system to maintain the symbolic links for programs. When you first run
visudo, it will prompt a list of editors for you to choose from. Unfortunately, it doesn’t tell you how to change that setting again. The mostly happend situation is: you set it to “nano” at first, and can’t change it to “vi” at a later time.
Here is the command to bring up that list again:
Some people suggest using command
EDITOR=vi visudo, or
export EDITOR=vi; visudo, but it’s really annoying if you have to remember and type so many letters everytime you run this command